Tunable microwave switching



United States Patent 01 ice 3,516,031 Patented June 2, 1970 US. Cl. 333-98 Claims ABSTRACT OF THE DISCLOSURE A waveguide switch includes a movable inductive post having contiguous portions of different thickness which post passes through an opening in the waveguide broad Wall and is an extension of the inner conductor of an adjustable coaxial transmission line stub. The other end of the inner conductor is selectively connected to a short circuiting plate by a diode. When the latter conducts, the stub presents effectively an open circuit between the inductive post and the broad wall through which it passes to pass energy with minimum attenuation. When the diode is not conducting, the inductive post resonates with the effective capacity seen at the broad wall opening between the inner conductor and broad wall to stop energy with maximum attenuation. Adjusting the length of the stub is accompanied by movement of the inductive post to vary the ratio of thick post portion to thin post portion within the waveguide, and thereby the effective inductance, so that the center frequency of the stop band, when the diode is not conducting, tracks the center frequency of the pass band when the diode conducts.

The present invention relates in general to tunable mircowave switching and more particularly concerns a novel diode waveguide switch characterized by good electrical performance, a mechanical structure relatively free from complexity and facilitating tuning through a single adjustment over a relatively wide band so that the center frequency of the stop band corresponds to the center frequency of the pass band.

Microwave diode waveguide switches are well known in the microwave art. A typical diode waveguide switch comprises an inductive post in the center of the waveguide coupled to the top broad wall and extending through an opening in the bottom broad wall to form the inner conductor of a quarter wavelength coaxial transmission line. A diode in series with this inner conductor, when conducting, comprises means for interconnecting the inner and outer conductors so that there is elfectively an open circuit between the inductive post and the bottom broad wall substantially at the frequency at which the coaxial transmission line effective electrical length is a quarter wavelength. The waveguide then transmits energy at this frequency with negligible attenuation. When the diode does not conduct, the inductive post resonates with the effective capacity between the post and the waveguide broad walls to form effectively a series resonant circuit intercoupling the broad walls and thereby prevent the transmission of energy through the waveguide at the frequency at which this series resonance occurs. It is a relatively simple matter to design such a switch so that when the diode is cut off, the series resonance occurs at the same frequency at which the effective electrical length of the coxial transmission line is a quarter wavelength when the diode conducts. However, it is not infrequently desired to tune the effective frequency of such a switch. Although there exist numerous schemes for adjutsing the effective quarter wavelength of the coaxial transmission line so as to establish the center frequency of the stop band, the usual prior art schemes have been unable to achieve tracking of the center frequency of the stop band and the pass band with a simple mechanical adjustment.

Accordingly, it is an important object of this invention to provide an improved diode microwave switch.

It is another object of the invention to achieve the preceding object with a simple mechanical adjustment that facilitates tuning over a relatively wide frequency range while maintaining tracking of the center frequencies of the pass and stop bands.

It is a further object of the invention to achieve the preceding object with apparatus that is relatively easy and inexpensive to fabricate, easy to adjust and capable of operating reliably for relatively long periods of time.

It is a further object of the invention to achieve the preceding objects with a simple mechanical adjustment capable of being adjusted by a relatively unskilled person to achieve the desired electrical preformance.

SUMMARY OF THE INVENTION According to the invention means define a waveguide. Means define a conducting post extending between the walls of the waveguide having portions of different cross section and forming the inner conductor at one end of a coaxial transmission line having its outer conductor secured to one of the walls to form a stub. Means including a unilaterally conducting device couple the inner and outer conductors of the coaxial stub so as to selectively allow and prohibit propagation of microwave energy through the section in the waveguide including the post. Means are provided for selectively adjusting the effective electrical length of the electrical stub so that such adjustment is accompanied by a change in the cross sectional area of the post through the section of the waveguide embracing the post so that the center frequency of the stop band tracks that of the pass band as the elfective electrical length of the stub is adjusted. Means are provided for selectively biasing the unilaterally conducting device to render it selectively conductive and nonconductive.

Numerous other features, objects and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawing in which:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a combined block pictorial schematic circuit diagram representing the equivalent circuit of an embodiment of the invention;

FIG. 2 is a view partially in section of a diode waveguide switch according to the invention illustrating mechanical details of a preferred embodiment of the invention;

FIG. 3 is a top view of the embodiment of FIG. 2;

FIG. 4 is a sectional view through a tuning element according to the invention; and

FIG. 5 is a sectional view through a choke element in the preferred embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With reference now to the drawing and more particularly FIG. 1 thereof there is shown a combined block diagramatic schematic circuit diagram of an embodiment of the invention helpful in understanding the principles of operation. A rectangular waveguide 11 has an upper broad wall 12 and a lower broad wall 13. Adjustable inductive means 14, typically a movable inductive post having contiguous sections of different diameter, is coupled to upper broad wall 12 by a low impedance coupling link, such as a choke joint 15, or a capacitive coupling and direct coupled to controlled biasing source 16. The other end of variable inductance 14 is connected to diode 17, typically by an extension of the inductive post forming the inner conductor of the coaxial stub 21. A shorting stub or plate 22 connects diode 17 to outer conductor 23 so that when diode 17 is rendered conductive by controlled biasing source 16, there is eifectively an open circuit between the inner conductor portion 23 and lower broad wall 13 in the plane of the lower broad wall. Inductance 14 is then effectively open circuited so that energy may be propagated through the waveguide 11 with negligible attenuation. Preferably the structure is so arranged that the shorting stub 22, the crystal 17, the inner conductor portion 23 and the variable inductance 14 are fixed relative to one another so that movement of the short circuiting stub, crystal and post assembly both changes the electrical length of the stub and varies the inductance 14 so as to establish the center frequency of the stop band the same as the center frequency of the pass band while permitting simultaneous adjustment of these center frequencies over a relatively broad frequency range with but the single mechanical adjustment.

When diode 17 does not conduct, there is an effective capacity 25 between inner conductor portion 23 and broad wall 13 in the plane of lower broad wall 13 which, together with whatever effective capacity choke joint 15 may introduce to upper broad wall 12, presents effectively a series resonant circuit between upper broad wall 12 and lower broad wall 13 that effectively interconnects these broad wall at this resonant frequency to stop the propagation of energy at this resonant frequency through waveguide 11.

Referring to FIG. 2, there is shown a side view of a preferred 3-stage embodiment of the invention with the first stage shown in section through a longitudinal plane passing through the midlines of the broad walls to illustrate mechanical details. Corresponding elements are identified by the same reference symbols throughout the drawing where applicable. Since each of the stages 31, 32 and 33 are identical, the detailed description which follows shall primarily be related to stage 31.

The member 14 comprising the inductive post includes an upper portion 34 of smallest diameter or the same diameter as intermediate portion 35 and a lower portion 36 of largest diameter. This member is supported by a con tact assembly including spring fingers 37 for establishing good contact during sliding and a threaded base member 41 screwably supported in the inside threaded outer' 'conductor 24. Thus, as member 41 is rotated, inductive post member 14 translates along its axis so that the ratio of wide diameter portion 36 to narrow diameter portion 35 changes to produce the electrical effects described above. Once member 41 is adjusted, a locking plug 42 may be inserted to lock member 41 into the desired position.

The upper portion 34 passes through choke joint 15 comprising a screwably adjustable choke housing 43 that rides on the threaded inside of outer conductor portion 44 with an annular load element 45 helping to further reduce leakage radiation. A plug 46 of insulating material is formed with an opening through which upper portion 34 may slide and screws into the threaded inside portion of outer conductor 44 to keep the annular load element 45 in position. Details of choke joint assembly 43 are set forth below in connection with the description of the sectional view in FIG. 5. A retainer cap 47 is screwed to the threaded end of upper portion 34.

Three set screws 51, 52 and 53 are seated in upper broad wall 12 along the centerline of the waveguide and introduce controllable trimming impedances that help suppress the propagation of undesired energy.

Upper broad wall 12 also carries a contact block midway between stages, such as contact block 54 seen in FIG. 2, with a portion shown in section to illustrate details of the fastening to the broad wall. Each contact block carries a spring contact that rides against upper portions, such as 34 and 34", and receives a lead at an input contact screw 55 from the controlled biasing source input terminal 56. Contact block 54 is seated upon a thin insulating wafer 56' and secured to upper broad wall 12 in insulatedly spaced relationship by means including insulating spacer 56. Screws 57 and 58 are seated in insulating collars 61 and 62, respectively.

The housing position is represented by the dashed dotted outline 64 and carries the biasing input terminal 56 shown as a conventional coaxial connector that may receive a signal from controlled biasing source 16.

Referring to FIG. 3, there is shown a top view of the assembly of FIG. 2 to illustrate the interconnection arrangements. The inner contact 65 of coaxial input terminal 56 is connected by a lead 66 to terminal block 67, terminal block 67 being essentially the same as terminal block 54. A lead 68 interconnects terminal blocks 54 and 67. Spring contact 59 interconnects upper portions 34' and 34 while an essentially identical spring contact 71 interconnects upper portion 34 and 34. This interconnecting arrangement establishes essentially identical biasing conditions on the diodes in stages 31, 32 and 33 at any given time.

Referring to FIG. 4, there is shown a longitudinal sectional view of the assembly 41 relative to certain other elements illustrating constructional details of a preferred embodiment of the invention. The upper portion of element 14 is shown in contracted form. The bottom end of the lower end of element 14 is formed with spring fingers 72 for firmly establishing electrical contact with one electrode of diode 73. An annular insulating element 74 coaxially surrounds and supports fingers 72 about the axis of the assembly and snugly fits within the annular shoulder 75 of element 41. The lower end of element 41 is hollow and internally threaded as shown. A contact disc 76 formed with an opening 77 for accommodating the other electrode of diode 73 is held in place by threaded plug 78. Screwing up plug 78 tightly supports the assembly firmly. An insulating washer of Teflon, or other suitable material, 81 surrounds intermediate portion 35 and rests on the top of lower portion 36 and functions to prevent the bias from being shorted out if the assembly is tuned to a higher frequency than it was designed for. That is to say, the washer 81 prevents choke element 43 and the top of lower portion 36 from being in conductive contact.

Referring to FIG. 5, there is shown an axial sectional view through choke element 43. Element 43 includes an externally threaded sleeve 82 formed with a top annular shoulder 83. An internal insulating element 86 of stycast coaxially surrounds the element axis and element 84 provides an opening through which element 14 may slide while being insulatedly separated from those metal elements at the same D-C potential as the waveguide walls. Insulating element 84 passes through the top hole in annular shoulder 83 to prevent intermediate portion 35 from shorting to annular shoulder 83. A conducting sleeve 85 separates insulating element 84 from annular insulating element 86 and is formed with a lower annular base 87 in contact with the outer wall of sleeve 82.

In operation choke joint element 43 is typically adjusted so that with the diodes conducting at the center of the operating frequency with the upper broad wall 12 and lower broad wall 13 effectively electrically interconnected, attenuation is a maximum. Screws 51, 52 and 53 may also then be adjusted to maximum attenuation.

Thereafter, the center frequency of the pass band and stop band may be simultaneously adjusted merely by screwing element 41 in and out in each of the stages. Although there may be many applications where stages 31 and 32-and 33 are tuned to the same frequency, there may be numerous applications where staggered tuning may be desirable and advantageous. In such a case each of the stages may be tuned to a specific design frequency by separate single frequency adjustment. Thereafter, the overall stop band and pass band response may be optimally adjusted by energizing the input from a sweep frequency generator and observing the output response with a suitable detector feeding a suitable oscilloscope display.

The specific structure described is by way of example for illustrating the best mode now contemplated for practicing the invention. Variations of this specific structure too numerous to mention may be practiced by those skilled in the art to cause the pass band and stop band center frequencies to track in a predetermined manner within the principles of the invention.

In a specific embodiment of the invention waveguide 11 was dimensioned to propagate TE (principal mode) energy in the C band with an internal width of 1.872 inches and an internal height of .872 inch. The inside diameter of outer conductor 44 was .150 inch, and the thickness of insulating sleeve 35 .012 inch. The outside diameter of portion 31 and of intermediate portion 32 was .125 inch. The outside diameter of lower portion 33 was .200 inch and its length 1.170 inches. Crystal 17 was a PIN diode. Shorting stub 22 was made of copper metal. The length of outer conductor 24 was 1.250 inches and its inside diameter .750 inch. Insulating wafer 34 was made of Teflon. In the preferred form of the invention there were three such sections spaced wavelength apart along the length of the waveguide and each adjusted to produce minimum attenuation at the selected center frequency with diode 17 closed and maximum attenuation at this center frequency with diode 17 open. This tuning could be accomplished over a frequency range of 4.4-5.0 gHz. with a maximum insertion loss when conditioned to pass energy of 0.7 db and a minimum attenuation of 95 db when conditioned to stop energy.

'It is evident that those skilled in the art may now make numerous uses and modifications of and departures from the specific embodiment described herein without departing from the inventive concepts. Consequently, the invention is to be construed as limited solely by the spirit and scope of the appended claims.

What is claimed is:

1. Waveguide switching apparatus comprising,

means defining a waveguide,

means defining a conducting post extending between opposed wall portions of said waveguide,

said conducting post having first and second portions of different cross section within said waveguide and a third portion passing through one of said wall portions and comprising one conductor of a transmission line stub having another conductor connected to said one wall portion,

means including a unilaterally conducting device for selectively intercoupling said one conductor and said another conductor at a point outside said waveguide to selectively allow and prohibit within pass and stop frequency bands respectively propagation of microwave energy through the section in said waveguide including said conductive post,

and means for relatively displacing said conducting post and said waveguide to selectively adjust the effective electrical length of said stub and change the volume of said conducting post within said waveguide to establish a predetermined relationship between the center frequency of said pass band and that of said stop band as said conducting post and waveguide are relatively displaced to shift said pass and stop bands.

2. Waveguide switching apparatus in accordance with claim 1 wherein the center frequency of said pass band coincides substantially with that of said stop band.

3. Waveguide switching apparatus in accordance with claim 2 wherein said stub is coaxial with said first conductor being the inner conductor thereof and said another conductor being the outer conductor. thereof.

4. Waveguide switching apparatus in accordance with claim 3 wherein said conducting post is formed with said first portion thinner than said second portion, with said second portion contiguous with said third portion where by shortening of the effective electrical length of said stub is accompanied by an increase in the volume of said coni ducting post within. said waveguide and a corresponding reduction in the effective inductance presented by said conducting post within said waveguide.

5. Waveguide switching apparatus in accordance with claim 4 and further comprising means for supporting said unilaterally conducting device within said stub so that upon nonconduction thereof the effective inductance of said conducting post coacts with the effective capacity between said post and said waveguide to form a series resonant circuit that establishes a low impedance path between said opposed wall portions at said stop band center frequency while upon conduction thereof said stub establishes an electrically effective high impedance path between said conducting post and said one wall portion.

6. Waveguide switching apparatus in accordance with claim 5 and further comprising,

means defining a choke joint around said conducting post near the other of said wall portions outside said waveguide for inhibiting leakage radiation,

said conducting post being insulatcdly separated from said waveguide and in conductive contact with one electrode of said unilaterally conducting device,

said unilaterally conducting device having another electrode in conductive contact with at least one Wall of said waveguide so that a biasing potential applied between said conducting post and said at least one wall may control the conductive state of said unilaterally conducting device.

7. Waveguide switching apparatus in accordance with claim 6 wherein said choke joint comprises an axially displaceable outer conducting sleeve surrounding an inner insulating sleeve that surrounds said conducting rod, an inner conducting sleeve that surrounds said insulating sleeve for most of the latters axial length and an annular dissipative element that surrounds said inner conducting sleeve for most of the latters length.

8. Waveguide switching aparatus in accordance with claim 6 wherein said third portion terminates with spring fingers defining a recess for gripping and electrically contacting said one electrode and said means for supporting said unilaterally conducting device includes,

an outer conducting sleeve formed with an inwardly extending annular shoulder,

an annular insulating sleeve between and abutting said inwardly extending annular shoulder and said third portion spring fingers,

a conducting disk formed with a central opening for snugly engaging said another electrode seated inside said central opening in conductive contact with said outer conducting sleeve against said annular shoulder and in engagement with the last-mentioned annular insulating sleeve,

said outer conducting sleeve being formed with conducting spring fingers surrounding said third portion spring fingers for slidably engaging said outer conductor in good conductive contact.

9. Waveguide switching apparatus in accordance with claim 8 wherein said choke joint comprises an axially displaceable outer conducting sleeve surrounding an inner insulating sleeve that surrounds said conducting rod, an inner conducting sleeve that surrounds said insulating sleeve for most of the latters axial length and an annular dissipative element that surrounds said inner conducting References Cited sleize gar mostgf the1att1terslengthi d th UNITED STATES PATENTS avegui e sw1 c 1ng appara us in accor ance W1 Claim 9 and further comprising, if}? 52 5 "3;;

1 d i at least another of said apparatus axially d1sp ace 5 2,781,500 2/1957 Armstrong 333 98 along the length of said waveguide by an odd number of quarter wavelengths at said stop band center frequency and further comprising,

means defining a terminal block seated upon the outside of said waveguide between contiguous ones of 10 3,164,792 1/1965 Georgcev.

3,392,354 7/1968 Plutchok 33398 3,092,781 6/1963 Kozul.

said apparatos, said terminal block including means for supporting a HERMAN KARL SAALBACH Primary Exammer spring contact insulatedly separated from said wave- L. ALLAHUT, Assistant Examiner guide wall portions in slidable conductive relationship outside said waveguide with said conducting 15 post of each said apparatus which spring contact 329161 comprises means for delivering a biasing potential to each contacted conducting post.

3,175,218 3/1965 Goebels 33398' 

